A capacitor is a device used in electronic circuits to store electrical charge. Electrical charge Q. is measured in coulombs and one electron has a charge of about 1.6.times.10.sup.-19 coulombs. Typically, the electrical charge is stored on the surfaces of separated plates (electrodes) immersed in an electrolyte. The plates are generally layered and may be planar or wound, e.g. rolled, as for example in a spiral roll. A dielectric layer of mechanical separator material, such as dielectric paper and the like, is arranged to maintain the plates separated from physical contact. The charge creates an electrostatic field which exists between the plates and therefore creates a potential difference, or voltage V, between the plates.
Capacitance C, is measured in farads which is defined as one coulomb per volt. In general, the capacitance of a device is determined by dividing the charge stored on the plates by the voltage the charge creates across the plates. By increasing the capacitance, a greater charge can be stored per unit volt.
Generally, capacitance can be increased in two ways; by increasing the area of the plates and by reducing the separation distance between the plates, such as by using very thin dielectric separators. In an electrolytic capacitor, capacitance is achieved on the anode (+) plate by electrolytically forming a thin layer of dielectric oxide on the surface and immersing it in an electrolyte solution which functions as the negative (-) plate.
The energy, in joules, stored in a capacitor equals 1/2CV.sup.2 wherein V is voltage. In some applications, it is desired to maximize the energy density of a capacitor package. One such application is in the biomedical arts, and especially in implantable devices such as medical defibrillators.
Defibrillators must be capable of supplying an intense burst of energy to the heart in a very short time. The battery power supply of a typical implantable defibrillator is incapable of producing this energy alone. Therefore, the battery is used to charge an electrolytic capacitor which is then used to deliver the energy to the heart. For obvious reasons, it is important to minimize the size of the capacitor which is usually the largest component in the defibrillator circuit. It is thus advantageous to use a capacitor having as high an energy density as possible. Wound configurations are generally highly reliable and more convenient to produce, but are subject to packaging inefficiencies.
One packaging inefficiency associated with wound capacitors is the presence of open space inherent in the use of winding mandrels. Wound capacitors utilize winding mandrels to tightly wrap the electrodes in a spiral, but upon withdrawal of the mandrel an open space is left in the center of the capacitor. The smaller the diameter of the mandrel the more anode material can be packaged in a defined cylindrical housing and the smaller the open space left from mandrel withdrawal.
Anode foil is generally available in two generic forms, a solid-core anode form wherein the core is considered effectively non-porous in nature and the surface is etched and generally comprises an oxide to maintain high voltage storage; and, a porous anode form which is generally deeply etched or otherwise treated to form pores through the foil, and which also generally comprises an oxide surface to maintain high voltage storage. Anode foils are generally brittle and tend to break when acutely bent, particularly when the bend is accompanied by tensile stress. Cathode foils are generally very thin flexible foils which bend easily without breaking.
Though small mandrel diameters are desirable for minimizing open spaces, practically speaking the minimum diameter of a commercially useful mandrel is limited by the brittleness of the anode foil. Breaking of an anode foil during assembly can cause both a disruption in the assembly process and a disruption in anode continuity, which can result in both lost manufacturing time and lost capacitor efficiency.
What is needed then, is a method of producing wound capacitors which permits reduction of mandrel diameter to decrease central open space and increase the overall capacitance efficiency of a cylindrical packaging housing.
One object of the invention is to provide a method for assembling a multiple anode capacitor which provides decreased open space within a cylindrical packaging container.
Another object of the invention is to provide a method for assembling a multiple anode capacitor which provides increased energy density within a defined container.
Still another object of the invention is to provide an extremely high energy density capacitor assembly especially suited for use in implantable medical devices such as defibrillators.
These and other objects of the invention will become apparent from the following recitation of the invention.